1. Field of the Invention
This invention is a device that amplifies optical signals without any intermediate transformation of the optical signal into an electronic signal. In the device the characteristics of a nonlinear medium are controlled in such a way that when combined with the transmission characteristics of the cavity, amplification of the optical signal results.
2. Background of the Prior Art
The development of the laser has influenced many areas of technology and in some has provided for devices far beyond the ken of the original pioneers in this field. So, for example, the laser has established a significant role in fields as diverse as medicine, civil engineering, semiconductor device fabrication, various manufacturing processes and of course general research and development. One major application, predicted by early workers in the field of laser technology, is still in a period of dynamic growth. This application involves the use of lasers in the field of communication. Although the obvious advantages of such an application in terms of greater bandwidth are widely recognized, problems of transmission and signal processing are formidable. In the field of transmission the development of optical fibers appears to have been a significant step along the road to effective transmission of optical signals. In the area of signal processing, some direct processing of optical signals through the use of nonlinear materials has been realized, and solid-state lasers will, in all probability, be useful as miniature light sources in future communications systems. However, the ultimate step to completely integrated optical circuitry has yet to be fully realized. Such integrated optics, comparable in scale and function to integrated electronic circuitry, would enable the engineer to work with optical signals in much the same way as one works with electrical signals today. Transformation of the optical signal to an electronic signal would occur only at the extreme terminals of the communication system, if at all. All amplifying and switching operations would occur with the signal in its optical form without intermediate electronic devices. The realization of completely integrated optical circuitry has been delayed for lack of a viable optical amplifier -- a device akin to a transistor but which would not require any intermediate electronic devices. This application discloses such an optical amplifier.
3. Description of the Prior Art
The inventive device utilizes the transmission characteristics of a nonlinear medium. These characteristics have been found by applicants to be controllable in such a way that a region of amplification is obtained.
Specific characteristics of nonlinear absorbers have long been known. Generally, light impinging on a linear absorptive medium will diminish in strength as it passes through the medium according to the formula EQU I.sub.L = I.sub.0 e.sup.-.beta.L ( 1)
here I.sub.0 is the initial beam intensity, I.sub.L is the intensity at any distance L in the medium and .beta. is the absorption coefficient embodying the absorption characteristics of the medium. .beta. is a known function of the impinging beam wavelength, for a given medium, and displays large increases in the region of ground state transitions. So, for example, in the case of a gas the absorption is very strong at a wavelength that connects one of the ground states with an excited state.
Despite the strong absorption of the impinging beam in the region of a transition, it has been found that as the beam intensity is increased a region is found where the absorbed energy approaches a maximum. This occurs when the intensity of the beam is sufficiently high to "excite" approximately half of the atoms to the upper state. At equilibrium this is the largest number of atoms allowed in the upper state at any given time. Under these circumstances any additional light impinging on the gas will not be absorbed. The medium is then said to be bleached or saturated. It has been previously shown that the transmission characteristics of a resonant optical cavity may be significantly altered when filled with such a saturable absorber. A simple heuristic argument will serve to demonstrate this fact.
An empty resonant optical cavity consists of two plane mirrors of high reflectivity placed at a distance L from one another. When light of intensity I.sub.0 impinges perpendicularly on one mirror an amount I.sub.0 T is transmitted into the cavity. Here T is the transmission of the appropriate mirror and is generally less than one. Once inside the cavity the light is reflected back and forth between the mirrors, some light being transmitted through the mirrors on each pass. If the distance between the mirrors is a multiple of one-half of the optical wavelength, then the cavity is said to be in resonance with the light, which light is then transmitted with little loss. This transmission is due to constructive interference of the light associated with each of the passes at the mirror surfaces. When this condition obtains, the intensity within the cavity is approximately EQU I.sub.C = I.sub.T /T (2)
here I.sub.C is the intracavity intensity and I.sub.T is the transmitted intensity. Since at resonance EQU I.sub.T .perspectiveto. I.sub.0 ( 3)
we obtain EQU I.sub.C = I.sub.0 /T &gt; I.sub.0. (4)
the light intensity within the cavity is greater than that incident on the cavity of the multiple intracavity reflections.
Now consider the cavity to be filled with a saturable absorber which saturates when irradiated with light of intensity I.sub.S. When light of intensity I.sub.0 impinges at right angles to one of the plates of the cavity the intensity transmitted into the cavity, EQU I.sub.TC = I.sub.0 T, (5)
decays exponentially with distance according to equation 1. As a result of this decay very little energy reaches the second mirror and efficient multiple reflections do not occur. If, however, the power transmitted into the cavity, I.sub.TC, is equal to I.sub.S EQU i.sub.tc = i.sub.s ( 6)
then the power entering the cavity is sufficiently high to saturate the absorber and any additional light will pass through the medium as though it were not there. Under these conditions, the cavity is said to be switched on. It is then clear from equations 5 and 6 that the incident intensity which will turn the cavity on is given by EQU I.sub.0 TURN ON = I.sub.S /T (7)
now consider that the cavity is turned on and that the incident intensity is lowered. We want to observe the intensity at which the cavity turns off. It is obvious from our prior discussion that the medium will cease to be transmitting when the internal cavity intensity goes below I.sub.S. The internal cavity intensity, however, is given by equation 4 EQU I.sub.C = I.sub.0 /T.
the turn-off condition then becomes EQU I.sub.C = I.sub.S = I.sub.0 /T (8) EQU i.sub.0 turn off = i.sub.s t. (9)
when the impinging intensity becomes less than I.sub.S T the cavity will turn off. While the above argument is only qualitative, comparing equation 7 with equation 9 demonstrates that, since T is less than 1, the incident intensity at which the cavity turns on is greater than the intensity at which the cavity turns off. Consequently, while the power within the cavity may be related in a single valued way to the transmitted power, the relationship between the input power and the cavity power, and hence that between input and output power is dual valued.
This bistability was first disclosed by H. Seidel in U.S. Pat. No. 3,610,731 and was applied by A. Szoke in U.S. Pat. No. 3,813,605 to the production of short optical pulses with variable lengths. Szoke also describes in his disclosure applications similar to square wave amplification, inversion, and triggering. However, there is no indication in the prior art that this device can be operated in other than an absorptive bistable mode. Applicants have discovered that under certain operating conditions a primarily dispersive bistable device is realized. Since applicants' bistable device is primarily dispersive it displays significantly less loss than the absorptive bistable device. The instant applicants have also determined that under certain operating conditions the region of bistability degenerates into a single valued relationship with differential gain. A new device for amplifying light signals is then realized. This device arises from an improved understanding of the transmission characteristic of a resonant cavity filled with a nonlinear medium. The essential elements of this improved model are described below.